This invention relates to automatic tint correction in a TV receiver and more particularly circuitry for modifying hue in the region identified with flesh tones without introducing color saturation errors.
In conventional analog TV receivers, color demodulators recover quadrature-related color mixture signals e.g., I and Q or (R-Y) and (B-Y), by synchronous demodulation with a reference signal phase locked to the color subcarrier. The phase angle of the reference signal relative to the subcarrier determines the tint or hue of the image ultimately displayed. To correct the tint for user preference, etc., apparatus is generally provided to alter the phase of the reference signal relative to the subcarrier. In general, slight errors in hue or tint are not detectable by the observer since there is no reference based on previous information with which the viewer can make a comparison and, therefore, the tint adjustment is relatively non critical.
TV viewers are, however, particularly critical of displayed flesh tones. To accommodate this viewer sensitivity, TV receivers include auto flesh circuits which rotate the chrominance vector toward the flesh tone axis (e.g. the I color mixture signal axis) whenever the chrominance vector is close to the flesh tone axis. Rotation may be controlled by monitoring the relative magnitudes of the color mixture signals and generating a signal proportional to their ratio. This signal is then used to instantaneously adjust the phase of the reference signal applied to the synchronous demodulators so as to rotate the chrominance vector toward the flesh axis, whenever the ratio of the color mixture signals is in a range indicating that the currently received signal describes a hue that is near flesh color.
In TV receivers which are designed to digitally process video signals, baseband video from traditional analog IF circuitry is converted to digital format (PCM) by an analog-to-digital converter. In these receivers, static tint correction may be accomplished by adjusting the phase of the sampling clock of the analog-to-digital converter relative to the phase of the subcarrier. This method of tint control is similar to the phase adjustment of the reference signal applied to the synchronous demodulator in the aforedescribed analog receiver.
Dynamic tint control, as for flesh correction, generally cannot be achieved by phase modification of the sampling clock if tint is determined from the digitized signal. Consequently, it is advantageous to perform dynamic tint correction by manipulation of demodulated color mixture signals. This general form of tint correction has been considered in the past, but has not provided sufficiently good performance to achieve wide acceptance in commercial TV receivers.
R. F. Worden in U.S. Pat. No. 3,873,760 entitled "Flesh Tone Correction Using Color Difference Signals" and N. W. Bell in U.S. Pat. No. 3,536,827 entitled "Color Hue Error Correction Apparatus" illustrate and describe alternative apparatus for effecting tint correction by manipulation of the color mixture signals after demodulation. These systems scale the respective color mixture signals and add portions of the color mixture signals to each other to produce "corrected" color mixture signals, the vector sum of which describe a chrominance vector rotated relative to the received chrominance vector. The Worden and Bell systems are relatively simple to implement with analog circuits. However, the vector summation process involved in these same systems require successive multiplications of one variable quantity by another, the successive products being produced at the video sample rate. Fast binary multipliers are not attractive to implement in a TV receiver environment. There is also a tendency to reduce color saturation with this form of tint correction.
G. L. Caprio et al, in their U.S. Pat. No. 3,852,807 entitled "Automatic Hue Control Circuit", describe a simplified circuit for automatic flesh correction which operates only on the I and Q color mixture signals. The I and Q color mixture signals are compared to determine the relative phase of the signal with respect to the I or flesh color axis. When the phase of the chrominance signal is within a predetermined range of the I axis, e.g. within .+-.15 degrees) the supply of Q color mixture signal to the RGB matrix is completely attenuated. This effectively rotates the chrominance vector into alignment with the I axis, tending to produce true flesh hue throughout the predetermined range for tint correction. But unless hue correction is exercised over a narrower range than is normally desired, color saturation is reduced to an unacceptable degree.